0264-A2

Bending Properties of Laminated Veneer Lumber: Effect of Keruing (Dipterocarpus spp.) Reinforcement and Cyclic-Boil-Dry-Treatment

H'ng Paik San ¹ & Paridah Md. Tahir ²

Abstract

Low density wood such as Pulai (Alstonia spp), Sesendok (Endospermum spp) and Kekabu Hutan (Bombax spp) have never been regarded as structural material due to their inferior strengths. Converting these timbers into LVL and reinforcing them with stronger timber could turn them into much sought after materials. These paper discusses the effects of incorporating Keruing veneers into LVL panels made from low density wood. The exterior grade LVL comprised 11-ply and 15-ply panels fabricated using four tropical hardwood, i.e., Keruing, Pulai, Sesendok and Kekabu Hutan. The Keruing veneers were located at the surface and the low density wood were arranged as core. Phenol formaldehyde (PF) resin was used as the binder. The LVLs were subjected to cyclic boil-dry test according to Standard Boil PS1-74. The bending properties and percent delamination were determined according to the Japanese Agricultural Standard for Structural LVL (1993) before and after the test. Result shows that by incorporating low density wood with Keruing veneers, both 11-ply and 15-ply LVL panels achieved the minimum requirements for various grades stipulated in the JAS for Structural LVL Standard (1993). At the same panel thickness, 15-ply LVL panels shows a better performance compare to those of 11-ply LVL. Presence of Keruing veneers as surface layers significantly increased the strength of the LVL panels. The higher the number of Keruing veneers, the stronger were the panels. All panels passed the delamination test stipulated on the JAS for Structural LVL (1993). Kekabu Hutan as a core layer experience the highest delamination, followed by Pulai and Sesendok (6.43%, 4.56%, 4.00% in 11-ply and 2.40%, 1.52%, 1.28% in 15-ply LVL respectively). The cyclic boil-dry aging test reduced the strength of the panels significantly particularly those having higher Keruing veneers. Combining Keruing and the low density wood veneers in LVL fabrication gave greater strength and more stable material.

Introduction

In Malaysia, only about 400 species of Dipterocarps have been commercially exploited (Chin, 1996). Many of the less popular timber species are usually marketed as mixed hardwoods species. In order to achieve effective and optimum utilisation of timber resources, efforts to utilise more low density and lesser-used timber species are encouraged. A typical example is converting these low-density timber into a more competitive structural material such as LVL. Wong (1995) reported that mechanical properties of lower density LVL could be improved by reinforcement with high density wood. This could be done with the incorporating of lower density wood species as core layer and high density wood as surface layers, where lighter weight end product can be produced without compromising its strength.

A number of investigations have been carried out in many developed countries on the mechanical properties, physical performance and durability of LVL materials. It is known that mechanical properties and engineering performance of LVL are affected by many factors, and veneer thickness is recognize as one of the important factors that influence the strength properties of LVL. According to Laufenberg (1983), thicker veneer usually has deeper lather checks, thus induces shear failure which gives rise to low strength uniformity and low shear strength in LVL.

Investigations on the strength performance of LVL fabricated from low density tropical hardwood species under dry condition have been reported (Wong, 1995). The work described here is an extension of these studies where the LVL were subjected to a cyclic boil-dry test prior to the testing.

The main purpose of this study was to evaluate the flexural strength behavior of LVL produced from low density woods reinforced with Keruing veneers. The effect of veneer thickness on the flexural strength of the LVL was also examined. The LVLs were subjected to Cyclic Boil-Dry Test where the effects of timber species and veneer thickness on the strength retention were analysed .

Methodology

LVL of 11-ply and 15-ply panels were manufactured using three low density tropical hardwood species i.e., Pulai (Alstonia spp), Sesendok (Endospermum spp) and Kekabu Hutan (Bombax spp) as core layer and Keruing (Dipterocarpus spp.) species as surface layers. The logs were peeled into 2.6 mm (to make 15-ply LVL) and 3.6 mm (to make 11-ply LVL) thick veneers. The veneers were kiln-dried to approximately 7% moisture content. Panels size of 1200 mm width x 2400 mm long x 38 mm thick size, were manufactured in a commercial plywood mill. Phenol Formaldehyde (41.5% solids) resin was used a binder. The 11-ply LVL comprised 4 and 6 pieces of Keruing veneers (2 and 3 veneers on each surfacee, respectively). Whilst the 15-ply LVL had 2 and 4 veneers on each surface or a total of 4 and 8 veneers respectively (Figure 1a-d).

The LVL panels were then cut to size according to the Japanese Agricultural Standard for Structural Laminated Veneer Lumber (1993) for determining both the flatwise and edgewise bending strengths before and after cyclic boil-dry treatment.

Figure 1a: 11-ply LVL with two Keruing veneers located on both surfaces. Core layers were made up of either Sesendok, Pulai or Kekabu Hutan.

Figure 1b : LVL 11-ply with three Keruing veneers located on both surfaces. Core layers were made up of either Sesendok, Pulai or Kekabu Hutan.

Figure 1c: 15-ply LVL with two Keruing veneers located on both surfaces. Core layers were made up of either Sesendok, Pulai or Kekabu Hutan.

Figure 1d: 15-ply LVL with four Keruing veneers located on both surfaces. Core layers were made up of either Sesendok, Pulai or Kekabu Hutan.

Twelve 90 mm width x 38 mm thick x 874 mm length sized specimens were cut from each LVL panel and were divided into two groups. (1) Test in dry condition and (2) test after cyclic boil-dry test according to Standard Boil PS1-74 (7). The specimens were destructively tested in both flatwise and edgewise bending according to JAS for Structural Laminated Veneer Lumber (1993).

In the cyclic Boil-dry treatment, the specimens were submerged in boiling water for 4 hours, then dried at 145 ⁰ F for 20 hours. The step was repeated five times.These specimens were conditioned at 65_5% TH and 21_2 ⁰ C.

Static bending test was performed on each specimen; in dry condition and after 5 boil-dry cycle. Delamination percentages were measured on the later and strength retention was calculated. The MOR and MOE were calculated as follow:

Calculation of Flexural Strength

where:

MOR = Modulus of Rupture N/mm ²
MOE = Modulus of Elastic N/mm ²
P_ult = ultimate total load, N
P_pl = load at proportional limit, N
__pl = deflection at proportional limit, mm
I = second moment of area, mm ⁴ (bh ³ /12)
L = length of span, mm
a = distance between the reaction point to the nearest loading point, mm
b = measured beam width, mm
h = measured beam depth, mm

The density and moisture content for each of the tested bending specimen were determined.

The percentage of delamination was calculated by using the equation below:

Result and Discussion

The densities for the six types of 11-ply and 15-ply LVL studied were tabulated in tables 1. The LVL comprised of 15 layers had the higher density compared to those LVL comprised of 11 layers. Generally, LVL comprised of Keruing and Pulai veneers had the lowest density among the group. It was observed that decreasing the number of Keruing veneers in the LVL greatly reduced the board's density.

Effect of Presence of Keruing Veneers

The result of flatwise modulus of rupture (MOR) and modulus of elasticity (MOE) for all types of LVL tested under normal condition are shown in Figure 5-8. In addition, the Least Significant Different (P_0.05) was performed to determine if there are significant differences among the mechanical properties of LVL produced from different number of Keruing veneer layers for LVL 11- and 15-ply.

Figure 5: Presence of Keruing veneers effect on the MOR for LVL 11-ply

Figure 6: Presence of Keruing veneers effect on MOR LVL 15-ply

Figure 7: Presence of Keruing veneers effect on MOE of LVL 11-ply

Figure 8: Presence of Keruing veneers effect on MOE LVL 15-ply

It is evident from the Fig. 5 to 8 that presence of Keruing veneer on the low density LVL has a significant effect on the mechanical properties of the LVL in both 11- and 15-ply. The number of Keruing veneers increase from 2 to 3 layers on each surface layers for all types of LVL 11-ply increased the MOR and MOE value from 5% - 30% and 10 % - 15%, respectively. The highest increment of MOR and MOE was observed for the LVL produce from Pulai species as core layer (30.8% for MOR and 12.0% for MOE). While, the MOR and MOE for LVL 15-ply show an increment of 32% - 52% and 12% - 44%, respectively. The highest increment in term of MOR and MOE value were observed in the LVL produced from Sesendok as core layer in the 15 layers LVL. As shown in Table 2, there were significant difference (P_0.05) between the number of Keruing veneers presence on the LVL produced from three low density woods. These findings suggest that improvement of mechanical properties of LVL produced from low density woods can be achieved by incorporating with Keruing veneers as surface layers. The number of Keruing veneers presence in the layers of the LVL, increased the mechanical properties of LVL. LVL with higher Keruing veneers experienced higher mechanical performance.

Effect of Veneer Thickness

As shown in Fig. 9 and 10, the flatwise bending properties (MOR and MOE) of the LVL 15-ply were higher than those LVL 11-ply. Both LVL 11- and 15-ply were having the same number of Keruing veneers (2 Keruing veneers on each surface of LVL) and final board thickness (38 mm).

Figure 9: Effect of veneer thickness on the 38 mm thick LVL MOR

Figure 10: Effect of veneer thickness on 38 mm thick LVL MOE

Generally, the LVL with thinner veneer (15 ply) had the better mechanical performance compare to those thicker veneers (11-ply). Table 2 shows that there were significant different (P_0.05) for LVL 11- and 15-ply produced from Pulai and Kekabu Hutan in term of MOR and MOE. The LVL produced from Sesendok species as core layer show no significant different on the MOR values when thinner veneers were used.

The variation in the MOR and MOE for the LVL 11- and 15-ply for all the LVL types is mainly due to the final density of the LVL panels. The thinner the veneer the more glue will used to laminate the veneer together. As the result of that, the compaction effect of the veneers during hot pressing, as well as, the presence of gluelines between the plies increase the density of the thinner veneer LVL. This strongly suggest that by using thinner veneers, the LVL will exhibit higher strength compare to those thicker veneers in production of LVL.

Long term performance of LVL

Accelerated aging test is the method used for rapid evaluation of the resistance of wood base material to outdoor weathering condition. The accelerated aging test in this study which carried out with 5 cycles of samples submerged in boiling water for 4 hours, then dried at 145 ⁰ F for 20 hours is equal to the LVL place under expose environment for one years. After undergoing the accelerated aging test, it was observed that the LVL samples delaminated in the glueline and cracks occurred on the surface and edge of the sample. The strength reduction was observed for all types of LVL after accelerated aging test.

All the panels passed the delamination test (_10%) based on the JAS for Structural LVL (1993). Table 3 shows the percentage of delamination values for all types of LVL. From the table, the LVL produced from Kekabu Hutan as core layer gave the highest percent delamination for both LVL 11-ply and 15-ply.

Strength reduction was observed on the MOE and MOR for all types of panels after undergoing the accelerated aging test. The reduction values were ranged from as low as 0.19% to 10% for MOR and 0.76% to 21.21% for MOE for the LVL 11-ply and 0.57% to 18.77% for MOR and 22% to 27% for MOE for the LVL 15-ply. As shown in table 4, the highest strength reduction on MOR were observed for the LVL comprised of 3 Keruing veneers on each surface layers and Kekabu Hutan species as core layer (10%) and LVL comprised of 2 Keruing veneers on each surface layers and Pulai species as core layer (18.77%) for LVL 11-and 15-ply, respectively. While for the MOE, the highest reduction was observed for all types of 15 layers LVL. The mean strength reduction was 25%.

Conclusion

Overall, all types of panels in both LVL 11 - and 15-ply met the requirements for various stress and stiffness grades in the JAS for Structural LVL (1993). The number of Keruing veneers and wood species significantly affect the strength properties on both 11- and 15-ply LVL. The best species combination was shown by Keruing-Sesendok in both of 11-ply and 15-ply LVL panels. Presence of the Keruing on the low density LVL provides the strength to the boards. Furthermore, by using thinner veneers in producing LVL, higher strength values were observed. All the LVLs after accelerated aging test had the strength reduction from as low as 0.2 % to the highest 27% for both MOR and MOE.

References

Anon. 1990. A Structural Timber Substitute. Wood Based Panels. Asia Pacific Forest Industries. Pp 50 - 53.

Japanese Agricultural Standard for Structural Laminated Veneer Lumber (1993) Japan: Ministry of Agriculture, Forestry and Fisheries.

Jenny Chin Siew Chin. 1997. Properties of LVL from Low Density and Lesser-Used Wood Species. Unpublished. Faculty of Forestry. UPM. Selangor, Malaysia

Jozsef Bodig & Benjamin A. Jayne. 1982. Mechanics of Wood and Wood Composites. Van Nostrand Reinhold Company Inc., New York. United State.

Kinajil, R.T; Eng, W.H and Zoe, M (1994) Glue Laminated Timber and LVL Applications in the Construction of Portal Frame Structures. In Proceedings of the National Seminar of Wood Based Panel Products, 23-24 November 1992, FRIM, Kepong.

Kunesh, R.H (1978) Micro Lam: Structural Laminated Veneer Lumber. Forest Products Journal 28(7):41-44

Wong, E.D. (1995) Properties of Laminated Veneer Lumber manufactured From Acacia Mangium Thinnings and Rubberwood (Hevia brasiliensis). Degree of Master Science. Thesis Report, Faculty of Forestry, University Pertanian Malaysia, Serdang.

¹ Faculty of Forestry,
Universiti Putra Malaysia
43400 Serdang. Selangor
E-mail: [email protected]
Tel: 603-89483164

² Faculty of Forestry,
Universiti Putra Malaysia
43400 Serdang. Selangor
E-mail: [email protected]
Tel: 603-89467187